Snap-fitting, non-dislocating hip joint socket implant

ABSTRACT

A hip joint socket implant for total hip replacement, can contain a modified surface configuration of its cement-fixable outer wall having small, widely dispersed cement spacer elevation members. The implant has a unique snap-fitted arrangement of the head-restraining marginal outlet of the acetabular component with posterior and anterior cut outs of the implant to avoid premature impingement against a prosthetic femoral neck, allowing a normal, non-dislocating range of unobstructed external and internal rotation of the hip, and a hood that is a marginally extended continuation of the superior one-half or so of the cup containment that is of a sufficient magnitude to reduce the overall dimension of the socket outlet to less than a hemisphere, which is especially pertinent to a conventional hip replacement acetabular cup prosthesis.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of patent application Ser. No. 09/160,746 filedon Sep. 25, 1998, now U.S. Pat. No. 6,136,034 (Oct. 24, 2000). Thebenefit of the same is claimed under 35 USC 120, and the specificationof that application is incorporated herein by reference.

FIELD AND PURVIEW OF THE INVENTION

In general, this invention concerns a one-piece, monolithic plasticenarthrodial type joint socket implant, useful in arthroplasty. Moreparticularly, it concerns certain innovative modifications in thepolyethylene acetabular prosthesis which is commonly used to replace thesocket side of the hip joint in total replacement arthroplasty,specifically designed for use without metal backing and employing asuitable cement such as a polymethylmethacrylate surgical cement tostabilize the replacement component. As an implant for conventionaltotal hip replacement, for example, it can include some significantfeatures: 1) a cementable outer surface with, for example, an array ofprotrusions, which set the limited depth to which the implant can beinserted into the acetabulum, preferably, the protrusions being of adove tail type, with there being present a series of grooves inaddition; 2) a snap-fitting inside cup containment, which extendsfurther than hemispherical about the corresponding implanted femoralball so that dislocation is less apt to occur, which also has cut-outsin critical areas of the cup that provide for not only a nearly if notfully natural range of motion but also protection from leverageddislocation.

EXPLANATION BY WAY OF BACKGROUND

Early on in the procedural history of total hip arthroplasty, allpolyethylene acetabular replacement components were designed withoutmetal backing and were routinely stabilized with cement. The outercemented surface of the component was provided with shallow,cement-adhering irregularities of some sort, usually in the form ofnarrow and shallow circumferentially oriented channel indentations. Thespherical prosthetic, head-containing dimension of the socket part ofthe implant was commonly limited to less than a hemisphere in theinterest of avoiding premature, motion-induced abutment of theprosthetic femoral neck against an otherwise overly extended marginaloutlet of the implant which would obstruct the functional range ofrotational joint motion and induce the associated propensity foranterior or posterior dislocation of the replaced joint.

The not infrequent occurrence of latent procedural failure owing toimplant loosening associated with degradation and separation of thecemented bone-prosthetic interface led to the introduction andsubsequent use of metal-backed polyethylene acetabular components in theearly 1980s for the intended purpose of ameliorating the incidence ofthis complication. The metal-backed part of the composite component wasmade available with either a porecoated acetabular interfacing surfacethat depended on latent bone growth to stabilize the prosthesis with thepatient's acetabulum or without a porous coated surface that requiredcement fixation to provide implant stability.

In the meantime, the reduction in the spherical marginal extension ofthe implant to less than a hemisphere is, of course, accompanied by acommensurate reduction in the overall depth of the prosthetichead-constraining socket, and, consequently, the replaced hips remainedvulnerable to the propensity for postoperative dislocation. Early in the1970s, an attempt was made to counter this complication by extending thespherical continuation of the socket margin a bit beyond a hemisphere,i.e., sufficient enough to provide a snap-fitted containment with theprosthetic femoral head. In this initial concept of a snap-fittedarticulation, the marginal extension was distributed evenly around thetotal circumferential margin of the socket outlet. As a consequence ofproblems that were associated with an articular constraining arrangementof this sort, the concept of a snap-fitted articulation was thenabandoned after a short period of use. However, the continuing problemof postoperative joint dislocation has been addressed more recently byother innovators who have designed the acetabular component with asuperiorly positioned, non-snap-fitted marginal extension for thepurpose of merely providing a structural block against dislocation.Nevertheless, in the art of metal-backed acetabular cups, a polyethyleneinsert in the multi-component, metal-backed acetabular cup was developedwith a snap-fitting arrangement with cut-out rims. See, BioPro, Inc.,“PSL Physiological Stress Loading Total Hip Replacement System Utilizingthe Horizontal Platform Supported Concept and the Cox Comb AcetabularComponent,” 1991; BioPro, Inc., “PSL Physiological Stress Loading TotalHip Replacement System Utilizing the Horizontal Platform SupportedConcept,” 1997. That insert, which has marginal cut-outs and wasdeveloped to eventually include a snap-fitting hood arrangement,contains, however, a smooth back save one round, centrally locatedbutton protruding therefrom, and a dovetail slot in an elevated rimportion, which smooth back rests against the smooth inside of the metalbacking and which button projects through a hole in the metal backingand which dovetail slot is snap locked into a corresponding male post onthe metal backing to assist in fastening the insert component to themetal backing component. Note, both BioPro, Inc. references, at pages 4(FIGS. H, I); and the 1997 BioPro, Inc. reference, at page 7 (CoxcombAcetabular Cups, illustration).

Drawbacks to the heretofore listed, variable methods of designing andfixation of an acetabular replacement component are multifactorial andare generally enumerated as follows:

First, the nature and distribution of the shallow irregularities on thecemented outer surface of the polyethylene component makes ittechnically difficult, if not impossible, to assure a maximallyefficient implant-stabilizing, bone-to-prosthesis cement mantle, i.e., acement mantle that is symmetrically distributed to provide aconsistently even and adequate thickness of cement throughout the totaldimension of the bone-prosthesis interfacing interval.

Second, acetabular components which are designed with a femoralhead-constraining marginal outlet that is reduced to less than ahemisphere are prone to postoperative dislocation. Although a relativelyinfrequent occurrence, postoperative dislocation of a replaced hip is aninordinately serious and disheartening complication. Aside from thesevere discomfiting and emotional effect of the ordeal on the patientand surgeon alike, this complication has a substantial adverse impactdue to the not insignificant costs associated with the required addedsurgical intervention and hospital care.

Third, polyethylene acetabular components that were designed with acomplete circumferentially extended snap-in marginal outlet produced yetan earlier premature stage of marginal abutment against the neck of thefemoral component, and an associated, commensurate reduction in therange of rotational motions, which in turn induce the propensity for a“leveraged-out” anterior or posterior joint dislocation, notwithstandingthe snap-fitted arrangement. Furthermore, repetitive, motion-inducedabutment between the femoral neck and the extended acetabular margin,particularly with rotary joint motions, resulted in erosive degradationof the repetitively traumatized, overextended margins of the acetabularcup outlet, consequentially producing yet another source for theproliferation and the associated detrimental effects of foreign bodydebris.

Fourth, cemented metal-backed acetabular components have had nodemonstrable salutary effect on the incidence of implant looseningcomplications.

Fifth, long-term experience with the use of uncemented porecoatedmetal-backed components, although shown to be effective in stabilizingthe implant, has demonstrated a substantive acceleration in the rate ofpolyethylene wear, and an associated significant increase in theincidence and severity of debris-induced periprosthetic osteolysis,occasionally with catastrophic consequences to the patient.

Sixth, the concept of a superiorly oriented, non-snap-fitted marginalextension of the implant, although reasonably effective in reducing theincidence of dislocation if properly aligned, is highlytechnique-dependent, i.e., relative to the appropriate rotationalpositioning of the extended head-restraining implant rim. This isspecifically relevant to the inability to predetermine with any absolutedegree of certainty as to whether a given hip is, or will be, vulnerableto an anterior versus a posterior joint dislocation. For instance, aninadvertent, erroneously suspected propensity for anterior dislocationthat is managed by a precautionary antero-superiorly positioned rimextension may result in a marginally unguarded posterior dislocation, orvice versa.

It would be desirable to overcome such drawbacks and provide an improvedsocket implant, the lack and need for which is of long standing in theart. It would be especially desirable to be able to apply improvementsto the human hip.

INTRODUCTION TO THE INVENTION BY WAY OF A SUMMARY AND ELABORATION

In general summary, the present invention provides an improvedenarthrodial type joint socket implant, useful in arthroplasty. In aparticular embodiment, illustrative of the invention with a complementof beneficial features, it pertains to certain specific modifications inthe design configuration of former and current other polyethyleneacetabular hip components, for example, for total conventional hipreplacement, which not only enhance the proficiency and longevity of thestabilizing bone-prosthetic cement mantle in the interest of extendingthe survivorship longevity of the acetabular component, relative to thetime to failure of the arthroplasty procedure, but also provide a securesnap-fitted articulation that produces the desired effect of guardingagainst the propensity for postoperative dislocation while at the sametime allowing for a normal functional range of motion of multi-planedjoint motion.

The invention addresses or overcomes the foregoing drawbacks.

A first part of the present invention, in general, concerns a modifiedsurface configuration of the cement-fixable outer wall of the componentthat is designed with small widely dispersed cement spacer elevationmembers to assure an optimally proficient implant-stabilizing cementmantle which is universally consistent in thickness and technicallyroutine to implant. The small spacing members may be of anyconfiguration and vary in height so long as they are sufficiently highto provide an appropriately thick cement mantle. A preferred embodimentconcerns rectangularly-oriented cement spacer elevation members in blockform, with dove tailed configuration, which may be machined in, andwhich, aside from assuring an appropriately distributed cement mantle,also provides a more secure, interdigited physical fixation with thecementing material, for example, polymethylmethacrylate.

A second general part of the invention, particularly pertinentespecially to conventional hip replacement acetabular cup prostheses,can concern an innovative snap-fitted arrangement of thehead-restraining marginal outlet of the acetabular component. Onefeature of this second general part of the invention pertains toposterior and anterior “cut outs” of the implant to particularly avoidpremature impingement against the prosthetic femoral neck, thus allowinga normal, non-dislocating range of unobstructed external and internalrotation of the hip. Although, if left uncompromised, a reduction in themarginal extension of the implant rim in these areas would ordinarilyprovoke a propensity for dislocation, this has been effectivelyaddressed by a second feature of this second general part of theinvention. This second feature pertains to a marginally extendedcontinuation of the superior one-half or so of the cup containment thatis of a sufficient magnitude to reduce the overall dimension of thesocket outlet to less than a hemisphere, which, as a consequence,reconstitutes the desired advantages of a snap-fitted articulationwithout incurring the aforementioned listed disadvantages associatedwith other attempts to provide a snap-fitted arrangement by the means ofa complete, uninterrupted circumferential extension of the marginal headconstraint.

Advantages of the aforementioned first general part of the presentinvention include the following: 1) the design-related improvement inthe physical mode of cement fixation and the technical ease of providingan appropriate, evenly distributed and universally thick cement mantleshould ameliorate, or at the least, decelerate, the process of implantloosening complications and prolong the time to failure of theacetabular component; 2) a commensurately thicker polyethylene componentcan be accommodated by eliminating the space-occupying thickness of ametal-backed container; and 3) a reduction in the rate of polyethylenewear and the associated incidence and severity of debris-inducedperiprosthetic osteolysis, a commonly cited cause of premature implantloosening and related complications. Also, 4) an ability to effectivelyemploy a socket implant made solely of ultra high molecular weightpolyethylene (UHMWPE), which, when cemented as the present invention canprovide and eliminating metal-to-bone contact, may reduce the rate orincidence of implant loosening due to the substitution of the metal withthe more resilient implant and cement, and consequent lessening of boneresorption at the implant-bone interface. In addition, 5) avoiding theuse of expensive metal-backed part in the component substantiallyreduces the overall cost of the replacement procedure.

Advantages of the aforementioned second general part of the inventionarise from the beneficial effect associated with the appropriatelypositioned marginal cut-outs and in addition the superior extension ofthe socket outlet in the prevention of dislocation. Aside from theprovision for a snap-fitted socket arrangement, the superiorly orientedrim extension produces an appropriately localized and extended “hood”that, in an by itself, provides an effective mechanical block againsteither antero-posterior or postero-superior dislocation, which are thecommonly encountered directional modes of this complication.Concurrently, the strategically positioned posterior and anteriorcut-out indentations augment the avoidance of dislocation by allowing anormal range of anterior and posterior hip rotations.

Numerous further advantages attend the invention.

DRAWINGS OF THE INVENTION IN BRIEF

The drawings form part of the specification hereof. With respect to thedrawings, the following is briefly noted:

FIG. 1 is a top plan view of an enarthrodial type socket implant of theinvention, embodied as acetabular cup, particularly adapted for a humancotyloid cavity, i.e., acetabulum, for conventional total hipreplacement.

FIG. 2 is a side plan view of the cup of FIG. 1.

FIG. 3 is a bottom plan view of the cup of FIG. 1, at a stage ofincomplete construction, looking into its ball-receiving cup.

FIG. 4 is a bottom view of the cup of FIG. 1, completed.

FIG. 5 is a sectional view of the cup of FIG. 1, taken along 5—5 asdepicted in FIG. 1.

FIG. 6 is a detailed sectional view of a portion of the cup as depictedin FIG. 5, taken from within the detail circle 6.

FIG. 7 is a side plan view of the cup of FIG. 1, receiving acorresponding conventional femoral ball from below, and stably cementedto suitable hip bone stock, which is shown in section.

FIG. 8 is an elevational isometric view of the cup of FIG. 1.

FIG. 9 is an isometric view of the cup of FIG. 1, from the oppositegeneral direction as depicted in FIG. 8, but slightly from the bottom.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The invention can be further understood by the present detail which maybe read in view of the drawings. The same is to be taken in in anillustrative and not necessarily limiting sense.

Although the joint of interest may be another enarthrodial joint, theinvention is described primarily with respect to and preferably is thehip. For example, the joint of concern may be the human hip joint.

With reference to the drawings, features generally common to socketimplants of the invention 100 (FIGS. 1-9) for a total conventional hipreplacement, which may be referred to as a head-receiving socket implantcup 100 include supporting body 101, and articular surface 110 which issmooth, and a cementable, mountable back surface 120. The articularsurface 110, when the cup 100 is suitably mounted in suitable receivingstock such as human cotyloid cavity bone 7 such as by cementing with amethacrylate surgical cement 8, mates in articulating, gliding contactwith the smooth external surface the femoral head implant whensurgically mounted (FIG. 7). Thus, the acetabular head-receiving cup 100can be provided with its outer, acetabular-facing surface 120 havingbuilt-in, dovetail-sided 121 elevated block spacers 122, which assureboth an evenly distributed and concentrically thick cement mantle and,concurrently, a secure cement interdigited prosthetic surface. Centralelevated block or button 123, which may be circular and with or withoutgrooves, also may be dovetailed. Grooveless, circumferential areas 124between the blocks 122 may be present. Grooves 125, for example, beingabout 0.020-inch in depth such as depicted with respect to the cup 100in detail by depth 125D (FIG. 6) with a 0.015-inch radius in the bottomthereof, also can help in cementing. Antero-inferior andpostero-inferior marginal extensions of the implant component 100 havebeen reduced by cut-out margins 126 to not only accommodate the anterioranatomical indentation of the socket containment and avoid anunsupported, non-contained peripheral “hang-over” of the cup rimanteriorly which could compromise the latent stability of the cup 100but also to minimize the potential for premature abutment of thepostero-inferior aspect of the rim against the opposing femoral neckwith external rotation and/or adduction of the hip. The marginal rimcut-outs 126 can be at 30-degree angles. Appropriate intraoperativetechnical placement of the marginal rim cut-outs 126 in relation to rimtop 127, which is angled, for example, with the rim top 127 at a30-degree angle, may be assured by identifying the designated superiorlypositioned rim of the component with a small, squared surface rimprojection 127P. Elevated or thickened rim portion 128 is also present.

Generally peculiar to the acetabular cup 100 for total conventional hipreplacement, in addition to the foregoing, is its snap-fitting ability.Such is generally provided through hood 129, which provides for agreater than hemispherical embrace of the corresponding femoralcomponent ball implant. However, as mentioned herein, with the cut-outs126, a stable, full range of motion is not compromised by thesnap-fit-providing hood 129.

The socket implant of the invention can be made of any suitable materialwhich, however, preferably is made of plastic. Advantageously, thesocket implant of the invention is UHMWPE.

In general, the cup 100 is made by known methods. In particular, theUHMWPE cup 100 is made by methods which include computer-assistedmachining and cutting of a UHMWPE blank, which, for example, may employa 0.375-inch ballnose cutter 50; a 1.528-inch length 51 may bereferenced (FIG. 3). Sterilization of the socket implant typicallyfollows.

Dimensions of the head-receiving cup may vary as needed for a particularapplication. For instance, with the UHMWPE acetabular head-receiving cup100 component as a prosthetic part of a total conventional hip joint,the cup 100 can be supplied in several femoral ball accommodatingdimensions, for example, in 28-mm and 32-mm dimensions, and, for furtherexample, the following dimensions listed in inches, with ±0.010-inchtolerances unless otherwise noted, may be encountered in totalconventional replacement joints sized in millimeters (mm) with theUHMWPE cup 100, for the 28-mm size ball-receiving cup 110 (FIGS. 1, 2 &5):

Feature/No. 46-mm 48-mm 50-mm 52-mm 54-mm 56-mm 58-mm 130 0.633 0.6330.633 0.633 0.633 0.633 0.633 131 0.292 0.264 0.231 0.192 0.152 0.1120.073 132 0.735 0.752 0.762 0.762 0.762 0.762 0.762 133 0.828 0.8670.906 0.945 0.985 1.025 1.064 134 0.886 ± 0.20 0.925 ± 0.20 0.964 ± 0.201.003 ± 0.20 1.043 ± 0.20 1.083 ± 0.20 1.122 ± 0.20 136 0.118 0.1180.118 0.080 0.080 0.080 0.080 138 1.198 1.209 1.215 1.215 1.215 1.2151.215 151 1.120 1.131 1.137 1.137 1.137 1.137 1.137 152 0.479 0.5070.549 0.577 0.606 0.649 0.678 153 1.639 1.718 1.796 1.883 1.963 2.0042.122 154 0.118 0.118 0.118 0.117 0.117 0.117 0.117 155 0.292 0.2640.231 0.192 0.152 0.112 0.073 Plunge Depth 0.272 0.278 0.276 0.283 0.2890.288  0.293;

dimensions 130, 133 & 134 are spherical radii, and dimension 153 refersto that dimension before cutting side angles; and the followingdimensions in inches, with ±0.010-inch tolerances unless otherwisenoted, may be encountered in total conventional replacement joints sizedin millimeters (mm) with the UHMWPE cup 100, for the 32-mm sizeball-receiving cup 110 (FIGS. 1, 2 & 5):

Feature/No. 50-mm 52-mm 54-mm 56-mm 58-mm 130 0.639 0.639 0.639 0.6390.639 131 0.214 0.186 0.152 0.112 0.073 132 0.824 0.835 0.841 0.8410.841 133 0.906 0.945 0.985 1.025 1.064 134 0.964 1.003 1.043 1.0831.122 ±0.20 ±0.20 ±0.20 ±0.20 ±0.20 136 0.118 0.080 0.080 0.080 0.080138 0.198 1.209 1.215 1.215 1.215 151 1.120 1.131 1.137 1.137 1.137 1520.549 0.577 0.606 0.649 0.678 153 1.796 1.883 1.963 2.004 2.122 1540.118 0.117 0.117 0.117 0.117 155 0.214 0.186 0.152 0.112 0.073 PlungeDepth 0.276 0.283 0.289 0.288 0.293

Common to these cups 100 are these dimensions (FIGS. 1-3, 5):

No. Value No. Value 139 10 ± 1-degree angle 145 60 ± 1-degree angle 14072 ± 1-degree angle 146 40 ± 1-degree angle 141 36 ± 1-degree angle 14730 ± 1-degree angle 143 27 ± 1-degree angle 148 30 ± 0.1-degree angle144 70 ± 1-degree angle 149 15 ± 1-degree angle 150 0.425 ± 0.005-inchdimension;

and these dimensions in the particularly sized cups 100 indicated abovecan be considered as typical.

Various other sized cups can be made in the practice hereof.

The cups of the invention can be implanted by generally known surgicalmethods. A suitable surgical cement is employed.

Conclusion

The present invention is thus provided. Various combinations andsubcombinations and features may be practiced with or without referenceto other combinations, subcombinations and/or features, alone or incombination, in the practice of the invention, and, moreover, numerousfurther adaptations and modifications can be effected within its spirit,the literal claim scope of which is particularly pointed out as follows:

I claim:
 1. A head-receiving acetabular cup for a ball and socket hip joint implant, said cup comprising a body; an articular surface; and a mountable back surface; said cup being monolithically made from plastic; said articular surface, when the head-receiving cup is suitably mounted in suitable receiving stock, mating in gliding contact with a smooth external surface of a corresponding ball joint head of a ball and socket head-containing femoral joint component when said corresponding component is also suitably mounted in suitable receiving stock; wherein: said body includes generally opposing, angled cut-out margins to anterio-inferior and postero-inferior marginal rim extensions of said body which can accommodate an anterior anatomical indentation of hip socket containment and engender avoidance of premature abutment of a postero-inferior aspect of the margin against an opposing femoral neck of said femoral joint component or an opposing femoral bone neck, with external rotation and/or adduction of the hip, which allow a normal, non-dislocating range of unobstructed external and internal rotation of the hip joint; the mountable back surface includes a plurality of small, widely dispersed cement spacer elevation members extending from the back surface to provide for an appropriately thick cement mantle; and which cup further includes a snap-fitting hood arrangement of the head-restraining marginal outlet of the acetabular cup, which includes a marginally extended continuation of a superior one half or so of cup containment that is of a sufficient magnitude to reduce an overall dimension of a socket outlet of the cup to less than a hemispherical embrace of the femoral head component.
 2. The cup of claim 1, further including an angled top margin between top portions of said generally opposing angled cut-outs.
 3. The cup of claim 1, wherein the plurality of small, widely dispersed cement spacer elevation members also are intended to assure an optimally proficient, implant-stabilizing cement mantle which is universally consistent and technically routine to implant.
 4. The cup of claim 1, wherein the angled cut-out margins are each at a 30-degree angle.
 5. The cup of claim 2, wherein the angled cut-out margins and the angled top margin are each at a 30-degree angle.
 6. The cup of claim 3, wherein the angled cut-out margins and the angled top margin are each at a 30-degree angle.
 7. The cup of claim 1, which is made of UHMWPE.
 8. The cup of claim 2, which is made of UHMWPE.
 9. The cup of claim 3, which is made of UHMWPE.
 10. The cup of claim 4, which is made of UHMWPE.
 11. The cup of claim 5, which is made of UHMWPE.
 12. The cup of claim 6, which is made of UHMWPE. 